Diatomic oxygen in its ground state is a paramagnetic molecule because it has a triplet ground state. Electronic excitation can produce either of two excited states, both of which are diamagnetic singlet states. The lower excited state, .sup.1 .DELTA..sub.g, has an energy of 22.5 kcal/mol above the ground state while the higher excited state, .sup.1 .SIGMA..sub.g, has an energy of 37.5 kcal/mol. Only the lower excited state has a long enough lifetime to be chemically active in solution and is normally referred to as "singlet oxygen", .sup.1 O.sub.2.
Singlet oxygen has been found to be a much stronger oxidizing agent than ground state molecular oxygen. Singlet oxygen has a calculated reduction potential of 1.7 V which makes it a better oxidizing agent (in non-acidic conditions) than ozone, hydrogen peroxide, sodium hypochlorite and chlorine dioxide. Thus, singlet oxygen can be used in a number of oxidation reactions such as synthesis of organic compounds, removal of humic acid from water, removal of phenols from waste streams, removal of cyanide from electroplating waste, oxidation of mercaptans in hydrocarbon streams and destruction of bacteria in various streams.
The usual method of generating singlet oxygen is by energy transfer from light and a photosensitizer. The role of the photosensitizer is to absorb the light and transfer its energy to the oxygen thereby forming singlet oxygen. The mechanism for producing singlet oxygen is well known in the art and the photosensitizers which can be used to produce singlet oxygen are also well known. Illustrative of these photosensitizers are rose bengal, methylene blue, eosin, chlorophyll, fluorescein, pyrene, acridine orange, porphyrins, phthalocyanines, etc.
The prior art teaches that these photosensitizers are usually used in a homogeneous phase; that is, the photosensitizer is dissolved in the reaction medium. This has the disadvantage that the photosensitizer must be separated from the reaction product. Even if separation is possible, complete separation is usually not achieved which means that fresh photosensitizer must be added to the fresh reaction medium. Since photosensitizers are expensive, the loss of photosensitizer may make the overall process uneconomical. Moreover, the effective concentration of photosensitizer which can be employed is limited not only owing to the increased difficulty of separating the photosensitizer from the products, but also owing to the fact that at higher concentrations the photosensitizers tend to form dimers and higher aggregates which reduce their effectiveness as photosensitizers.
One way to solve these problems is to carry out the photooxidation in a heterogeneous phase. Such a system is disclosed in U.S. Pat. No. 4,315,998 (see also Canadian patent Nos. 1,044,639 and 1,054,971). The '998 patent discloses chemically binding the photosensitizer to a polymeric material. The polymer used in the '998 patent is a modified crosslinked polystyrene polymer to which the photosensitizer is bound. The process used to attach the photosensitizer to the crosslinked polystyrene is a nucleophilic displacement reaction. However, crosslinked polystyrene has several disadvantages. One disadvantage is that crosslinked polystyrene is not transparent and thus only a portion of the photosensitizer is exposed to light. For example, if the polystyrene is in the shape of spheres which are placed in a column, only the outside spheres will be exposed to light, while the interior spheres will not. Therefore, reaction will only take place on the exterior of the column. Also, polystyrene has a tendency to swell and deteriorate in organic solvents.
Thus, there is a need for a polymer bound photosensitizer which utilizes all the available photosensitizers and is stable in organic solvents. Applicants have developed such a photosensitizer. Applicants' polymer bound photosensitizer consists of a cellulose acetate polymer to which is bound a photosensitizer. The photosensitizer is bound to the cellulose acetate by reacting a functional group (e.g., carboxylic acid group, sulfonyl chloride group, etc.) on the photosensitizer with a hydroxyl group on the cellulose acetate, thereby forming an ester linkage. Hydroxyl groups are present on the cellulose acetate because the cellulose acetate is not fully acetylated.
Comparing the cellulose acetate bound photosensitizer of the present invention with the polystyrene bound photosensitizer of the prior art shows several striking differences. First, the cellulose acetate polymer of the present invention is clear and lets light through whereas the crosslinked polystyrene of the '998 patent is opaque and does not let light through. Second, cellulose acetate neither swells nor deteriorates in organic solvents whereas the crosslinked polystyrene does swell in organic solvents. Third, cellulose acetate can be applied as a coating onto various solid supports whereas the crosslinked polystyrene cannot. Finally, the methods of attaching the photosensitizer to the polymer are different. The photosensitizer in the present case is attached to the polymer through an esterification reaction, whereas the '998 patent uses a nucleophilic displacement reaction to attach the photosensitizer. For all of these reasons, applicants' invention represents a significant improvement in the art.
Not only have the applicants bound photosensitizers to a polymer which has several advantages over polymer bound photosensitizers of the prior art, applicants are the first to discover a new use for the cellulose acetate bound photosensitizer. This new use involves oxidizing mercaptans which are present in various hydrocarbon streams with singlet oxygen generated from light, oxygen and a cellulose acetate bound photosensitizer. The conventional or prior art way of treating a hydrocarbon stream containing mercaptans is to first contact the hydrocarbon fraction with an aqueous alkaline solution, thereby transferring the mercaptans into the aqueous phase, followed by oxidation of the mercaptans with oxygen in the presence of a metal phthalocyanine catalyst. This process has the disadvantage in that it forms a waste alkaline stream which must be disposed. By using photooxidation, the disposal problems associated with the use of an alkaline solution are eliminated. Therefore, applicants' invention solves an important environmental problem related to the disposal of the alkaline solution.